Calculating days left until the next specified day

Question

In a scenario where you would need to calculate the next 'Billing date' if the DAY (2nd, 25th, etc) is known, how can you calculate the number of days left until the next bill payment?

Explanation:

Tom's bill gets generated on the 4th of every month

What's the best way/logic to calculate the days left until the next bill? For example, if today is the 28th of this month, the result would be 6 days left

What we know:

Bill Generation Date is known

Today's Date is known

What I've done so far:

int billingDay = 4; //The day the bill gets generated every month
DateTime today = DateTime.Today; //Today's date

How would I continue this to calculate the next billing date?

P.S: Sorry if this sounds lame, I just couldn't wrap my head around it :)

You might find this SO post useful: http://stackoverflow.com/questions/1607336/calculate-difference-between-two-dates-number-of-days — Matt Dalzell, Aug 09 '16 at 14:23
[DaysInMonth](https://msdn.microsoft.com/de-de/library/system.datetime.daysinmonth(v=vs.110).aspx) will help you at the first point. Just get the actual count of the days in the current month. Then `Today's date - actual count` and add the billingDay. — Christoph K, Aug 09 '16 at 14:24
Wow love all the responses. Thank you for taking the time everyone :) — Jay, Aug 09 '16 at 15:05

rory.ap · Accepted Answer · 2016-08-09T14:40:28.583

3

I think this works:

private int GetNumDaysToNextBillingDate(int billingDayOfMonth)
{
    DateTime today = DateTime.Today;

    if (today.Day <= billingDayOfMonth)
    {
        return (new DateTime(today.Year, today.Month, billingDayOfMonth) - today).Days;
    }
    else
    {
        var oneMonthFromToday = today.AddMonths(1);

        var billingDateNextMonth = 
            new DateTime(oneMonthFromToday.Year, 
            oneMonthFromToday.Month, billingDayOfMonth);

        return (billingDateNextMonth - today).Days;
    }
}

edited Aug 09 '16 at 14:40

answered Aug 09 '16 at 14:34

rory.ap

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1

I personally prefer this one, it seems to handle all the issues pointed out by the viewers. Thank you bro :) Thanks everyone else for your solutions as well! – Jay Aug 09 '16 at 15:05
You're welcome! Although it's more lines than some of the other answers, I think it's easier to understand which is important, too. It reads like a book. One-line statements can get confusing pretty quickly (especially when they wrap!). – rory.ap Aug 09 '16 at 15:11

Matias Cicero · Answer 2 · 2016-08-09T14:35:50.640

2

How about:

int billingDay = 4;
DateTime today = DateTime.UtcNow;
DateTime billing = today.Day >= billingDay
                   ? new DateTime(today.AddMonths(1).Year, today.AddMonths(1).Month, billingDay)
                   : new DateTime(today.Year, today.Month, billingDay);
TimeSpan left = billing - today;

edited Aug 09 '16 at 14:35

answered Aug 09 '16 at 14:27

Matias Cicero

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What if it's the end of the year? – rory.ap Aug 09 '16 at 14:28
@roryap Great observation. Fixed it. – Matias Cicero Aug 09 '16 at 14:28
1

what if today is third of a month? – trailmax Aug 09 '16 at 14:28
@MatiasCicero If the date is the 1st then the number of days is 3, not something like 34. Basically if today is before the billing day then you don't add one month. – juharr Aug 09 '16 at 14:31
he meant to say that you've hard-coded the date as the 4th. still, a great solution – MichaelThePotato Aug 09 '16 at 14:31
1

Run your code for `DateTime today = new DateTime(2016, 8, 3);` - if billing day is 4th, then you should get 1, but your code results 32. – trailmax Aug 09 '16 at 14:31
1

@trailmax Oh, I see what you meant! Will fix it – Matias Cicero Aug 09 '16 at 14:32
1

@trailmax Updated my answer – Matias Cicero Aug 09 '16 at 14:36
1

This is damp: `new DateTime(today.AddMonths(1).Year, today.AddMonths(1).Month, billingDay)` Sometimes one-liners aren't the best approach. – rory.ap Aug 09 '16 at 14:45
@MatiasCicero -- Just saying, brevity doesn't always equate to better. Most of time, sure. But I think "DRY" wins over "brief". – rory.ap Aug 09 '16 at 14:49
Thank you for taking the time everyone :) – Jay Aug 09 '16 at 15:04

D. Stewart · Answer 3 · 2016-08-09T14:46:10.413

1

This link might help you :

https://msdn.microsoft.com/en-us/library/system.datetime.daysinmonth(v=vs.110).aspx

What you can do is something like this:

        int daysUntilBill = 0;
        int billingDay = 4;
        DateTime today = DateTime.Today;
        if (billingDay > today.Day) {
            daysUntilBill = billingDay - today.Day;
        } else {
            int daysLeftInMonth = DateTime.DaysInMonth(today.Year, today.Month) - today.Day;
            daysUntilBill = billingDay + daysLeftInMonth;
        }

or slightly more concise

        int daysUntilBill = (billingDay >= today.Day) 
            ? billingDay - today.Day 
            : billingDay + DateTime.DaysInMonth(today.Year, today.Month) - today.Day;

This properly handles the year ending too, since it doesn't try to wrap around.

edited Aug 09 '16 at 14:46

answered Aug 09 '16 at 14:27

D. Stewart

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When using `DaysInMonth` you shouldn't use a hard coded year. – juharr Aug 09 '16 at 14:28
@D.Stewart no it does not. If you run your code with `DateTime today = new DateTime(2016, 8, 3);` you get 28. But billing day is on 4th of August - only one day away. – trailmax Aug 09 '16 at 14:34
Thank you for taking the time everyone :) – Jay Aug 09 '16 at 15:05

juharr · Answer 4 · 2016-08-09T14:47:22.527

First you need to determine if the current date is on or before the billing day and if it is just subtract the current day of the month. Otherwise you have to determine the next billing date in the following month.

public int DaysToNextBill(int billingDay)
{
    var today = DateTime.Today;

    if(today.Day <= billingDay)
        return billingDay - today.Day;

    var nextMonth = today.AddMonth(1);
    var nextBillingDate = new DateTime(nextMonth.Year, nextMonth.Month, billingDay)
    return (nextBillingDate - today).Days;
}

The only thing left to deal with is if billingDay is greater than the number of days in the current or following month.

Thank you for taking the time bro :) – Jay Aug 09 '16 at 15:05 — Jay, Aug 09 '16 at 15:05

Matt Wilko · Answer 5 · 2016-08-09T14:46:08.247

This uses a loop but is less prone to error as it takes into account month and year changes:

    int DaysUntilBilling(int billingDay, DateTime referenceDate)
    {
        int count = 0;
        while (referenceDate.AddDays(count).Day != billingDay)
        {
            count++;               
        };
        return count;
    }

You of course don't need to pass a DateTime in as an argument if you are always using today's date, but this helps to test that that for different inputs, you get the desired output:

int billingDay = 4;
DaysUntilBilling(billingDay, DateTime.Now); //26 (today is 9th Aug 2016)
DaysUntilBilling(billingDay, new DateTime(2016, 09, 03); //1
DaysUntilBilling(billingDay, new DateTime(2016, 09, 04); //0
DaysUntilBilling(billingDay, new DateTime(2016, 08, 05); //30
DaysUntilBilling(billingDay, new DateTime(2016, 12, 19); //16

Calculating days left until the next specified day

5 Answers5